Basically, when you've downloaded and installed the IME for your version of Microsoft Office and for the language that you want, you'll open up a new Word Document (for example). When you do, a small blue box should open up on your toolbar with the letters "EN" in it. Right click it and choose Japanese. Then you can choose from several options including Hiragana and Katakana. Just play around with it and you'll figure it out ^^. If you have any other questions, feel free to PM me.

Purveyor of the classics:

The Tale of Genji (currently)
The Kojiki (next in line)
The Houshin Engi (if I can find an English or Modern Japanese translation)